1. Field of the Invention
The present invention relates to magnetoelectric or magnetoresistive transducers and processes for producing the same and more particularly to a transducer having as a magneto-sensitive portion an InAs thin film with a resistance value which shows a very small temperature dependence and a process for producing the same.
2. Description of the Prior Art
Conventional methods for producing InAs Hall elements include various methods. For example, the first method includes the steps of preparing a single crystal of InAs, slicing and polishing it to obtain a thin material, and producing an InAs Hall element by using the thin material. The second method includes the steps of depositing an InAs polycrystal thin film on a substrate of mica, peeling it off from the substrate, bonding it to a substrate made of materials such as ferrite, and producing an InAs Hall element by using the InAs polycrystal thin film. The third method uses an InAs thin film, which has been grown on a GaAs substrate, for producing an InAs Hall element.
However, according to the first method, it is difficult to produce an InAs thin film in a constant thickness on an industrial scale, particularly it is very difficult to make the InAs thin film to a thickness of 1 .mu.m or less, thus failing to meet demand for mass production. The second method can provide an InAs thin film in a constant thickness but it involves formation of an insulating layer made of an organic substance as an adhesive between the thin film and the ferrite substrate. The organic substance layer as an adhesive is not suitable material for InAs Hall element which is operated at high temperatures exceeding 100.degree. C. because of the instability of it at high temperatures. Therefore, stable driving of this Hall element is impossible at high temperatures exceeding 100.degree. C. The Hall element produced by the third method, which does not involve the organic layer between the InAs thin film and the GaAs substrate, can be operated even at high temperatures such as exceeding 100.degree. C. However, it is known that because the thin film of InAs and the substrate of GaAs are different materials from each other, the single crystal of InAs in the vicinity of the interface with the GaAs substrate has many lattice defects due to misalignment of lattices with the GaAs substrate. A crystal lattice of the InAs near the interface with GaAs substrate is somewhat disorganized. Hence, when the InAs thin film was used as a Hall element, a large temperature dependence of the resistance was observed. It has a characteristics that the resistance value decreases at the temperatures above 60.degree. C. Hence, when used at a temperatures exceeding 100.degree. C. under a constant voltage driving, the Hall element using the InAs thin film generates heat due to the above-described decrease in resistance as the temperature of the Hall element increases. Since this heat generation causes the temperature of the Hall element to rise, the Hall element is accelerated as a feedback phenomenon. This causes a self-breaking type failure mode for Hall elements at constant voltage driving. This is a serious defect in driving Hall elements. In order to overcome this defect, it is required to change the temperature coefficient of the resistance which is negative at 100.degree. C. or higher to nearly null or positive.
Generally, it is possible to reduce temperature-dependent change in resistance of the InAs thin film by increasing electron concentration. In the case of an InAs thin film, its sheet resistance values suitable for Hall element use depends on the design conditions and input voltage values under which Hall elements are driven. Therefore, a lower limit value of the sheet resistance is determined, and this value restricts and determines the corresponding upper limit of the electron concentration. Within this limitation, it is expectable to reduce the temperature-dependent change in resistance in resistivity of the InAs Hall element or InAs thin film from room temperature to the vicinity of 100.degree. C. by merely increasing the electron concentration. However, it is impossible to reduce the temperature-dependent change in the resistance by increasing the electron concentration in the temperatures above 100.degree. C. This is because the temperature dependence in electron mobility dominates the temperature dependence in the resistance at high temperatures exceeding 100.degree. C. Accordingly, it has heretofore been found no technique that can reduce the temperature-dependent change in resistance of an InAs thin film having a thickness of 1.4 .mu. m or less at 100.degree. C. or higher. In other words, it is a novel technique to reduce temperature-dependent changes at 100.degree. C. or higher. The reduction of the temperature-dependent change in the resistance of an InAs thin film having a thickness of 1.4 .mu.m or less is important for producing practically useful Hall elements. This is because, an InAs thin film having such a small thickness formed on a substrate has different temperature dependence in electron mobility as compared with bulk InAs single crystal case, and behaviors of these temperature-dependent changes have not ever been understood.
In production of practical Hall elements, attempt to make small sized Hall element chips (0.4 mm square or smaller) in view of convenience of application and cost requirement causes the heat generation by consumption of electric power, heat concentration on a small sized Hall element chip and rising of the chip temperature. Then, the rising of the chip temperature leads to a self-breaking type failure. Therefore, it is necessary to reduce the temperature-dependent change in resistance, and no decrease in resistance with the increase in temperature is desired. However, this technology has not been achieved up to the present.